The studies of chronobiology on biological phenomena revealed the existence of a human internal mechanism ("biological clock") which controls biological events according to each function of the body. Most of the programmed and genetically fixed rhythms can be changed and synchronized by external factors. Starting from these data and from our previous studies which emphasised an intradian rhythm of the proteolytic blood coagulation and fibrinolytic systems we observed the behaviour of the fibrinolytic system in modified environmental conditions (continuous illumination and inversed day-night rhythm). Our experiments were performed on 180 Wistar rats both male and female, submitted to continuous illumination and inversed rhythm for 1, 3 and 5 weeks. We studied plasma fibrinolytic activity (PFA) through euglobulin lysis time (ELT) at three different moments of the day: 8:30, 18:30 and 8:30 the next day. The study revealed intradian variations of PFA regardless of the type and time of submission. Thus: continuous illumination for 1, 3, 5 weeks determined a decrease of PFA (prolonged ELT) during the second part of the day, less important at 5 weeks. Inversed day-night rhythm for 1 and 3 weeks determined the same intradian behaviour of PFA but less important than continuous illumination. PFA variations in our experimental conditions suggest an alteration of its biological rhythm.